This invention relates to a method and apparatus for detecting motion through a transparent enclosure, such as an electric utility meter. The method and apparatus includes a means of detecting and compensating for spurious information which might be generated as a result of defects or anomalies in the utility meter itself.
Specifically, the invention includes at least one light emitting diode (LED) which is directed onto the rotating disk of an electric utility meter. A photosensor is positioned to detect light reflected from the rotating disk. Each time the calibration mark on the disk rotates past the LED, the photosensor senses a pulse. By counting the pulses, the amount of electric current used during a given period of time can easily be calculated. Devices which operate in the manner described immediately above are known. For example, the Boldridge, Jr. U.S. Pat. No. 4,204,115, includes two spherical lenses positioned in the plane of the disk with an optical fiber cable coupling visible light from an incandescent source to one of the spherical lenses to focus light on the periphery of the disk and a second fiber optical cable transmitting light collected by the second spherical lense focused on the periphery of the disk to a photoresistor circuit. The Krogh et al U.S. Pat. No. 4,491,826, discloses an incremental digital converter for converting two-phase displaced signals of a pulse generator. The signals represent the position and direction of an object moved in two opposite directions. One of the functions of the digital converter is to compensate for errors which result from movement of the object in the wrong direction.
The Marshall U.S. Pat. No. 4,321,531, discloses a direction sensitive pulse initiator for a watt meter. A pair of sensors senses the presence of an indicator portion or portions on the rotating disk. Electrical signals are produced which are decoded to initiate clockwise rotation pulses and/or counterclockwise rotation pulses.
Prior art patents have also recognized that simply counting the number of times the calibration mark rotates past a sensing device is more difficult in application than in theory. One of the primary reasons for problems in this area is the fact that electric utility meters are usually inexpensively made and are subject to a variety of misadjustments and calibration errors. In addition, utility meters are subject to a substantial amount of abuse. The rotating disk portion of the meter may be bent or warped by uneven heating or damage during installation, service or repair. Furthermore, since these utility meters are often placed on the outside of buildings, corrosion and pitting of the metered disk can occasionally result. Furthermore, the disks are sometimes loosely mounted which can cause wobbles as the disk rotates. These defects will often cause reflections or gaps in reflections which are picked up by a sensor and, if not compensated for in some way, indicate that more revolutions have occurred than is actually the case. It does not appear that the prior art has recognized this particular problem, as such. Specifically, the prior art does not appear to acknowledge that while such defects are common, they are almost always regular, i.e., the same spurious pulses occur during each and every rotation of the disk. This principle has now been established and, accordingly, this application describes a method and apparatus for inexpensively detecting and compensating for such errors. Since it has now been determined that errors of the type described above appear regularly, it is possible to visually observe the rotation of the disk and determine if more than one pulse is generated during each rotation. For example, if three pulses are obtained during each rotation, one of these pulses is obviously caused by the calibration mark on the disk. The other two pulses might be caused by a number of error conditions, some of which are suggested above. By providing a counter and setting number "3" on the counter, the counter can be made to count down each time it senses a pulse. Each time the number "1" is reached, the counter outputs a signal which corresponds to a single full rotation of the meter disk. In this way, the meter disk need not be replaced or the error conditions otherwise corrected. It is enough to simply recognize that the error conditions are present and to compensate for them.